注解与反射

性能分析

普通方式>反射关闭检测>反射

package TestReflected;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//分析性能问题
public class Test08 {
    
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
    
        test01();
        test02();
        test03();
    }


    //普通方式调用
    public static void test01() {
    
        User user = new User();
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
            user.getName();
        }
        long endTime = System.currentTimeMillis();

        System.out.println("普通方法执行1000000次需要的时间：" + (endTime - startTime) + "ms");
    }


    //反射方式调用
    public static void test02() throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {
    
        User01 user01 = new User01();
        Class C1 = Class.forName("User01");
        Method getName = C1.getDeclaredMethod("getName", null);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
            getName.invoke(user01, null);
        }
        long endTime = System.currentTimeMillis();

        System.out.println("反射方法执行1000000次需要的时间：" + (endTime - startTime) + "ms");
    }


    //反射方式调用，关闭检测
    public static void test03() throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, IllegalAccessException {
    
        User01 user01 = new User01();
        Class C1 = Class.forName("User01");
        Method getName = C1.getDeclaredMethod("getName", null);
        getName.setAccessible(true);
        long startTime = System.currentTimeMillis();
        for (int i = 0; i < 100000; i++) {
    
            getName.invoke(user01, null);
        }
        long endTime = System.currentTimeMillis();

        System.out.println("反射方法，关闭检测执行1000000次需要的时间：" + (endTime - startTime) + "ms");
    }

}

通过反射操作对象

package TestReflected;

import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

//动态创建对象，通过反射
public class Test07 {
    
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
    
        //获得Class对象
        Class c1 =User.class;

        //构造一个对象
      // User user = (User) c1.getDeclaredConstructor().newInstance();
       // System.out.println(user);

        //通过构造器创建对象
        Constructor constructor = c1.getDeclaredConstructor(int.class, String.class);
        User user2 = (User) constructor.newInstance(18, "yan");
        System.out.println(user2);

        //通过反射调用普通方法
        User user3=(User)c1.newInstance();
        //通过反射获取一个方法
        Method setName =c1.getDeclaredMethod("setName", String.class);

        //invoke:激活的意思
        //（对象，""方法的值）
        setName.invoke(user3,"yan");
        System.out.println(user3.getName());


        //通过反射操作属性
        User user4=(User)c1.newInstance();
        Field name = c1.getDeclaredField("name");
        //不能直接操作私有属性，我们需要关闭程序的安全检测，属性/方法的setAccessible(true)
        name.setAccessible(true);  //
        name.set(user4,"yan2");
        System.out.println(user4.getName());

    }
}

用反射操作注解

package TestReflected;

//练习反射操作注解

import java.lang.annotation.*;
import java.lang.reflect.Field;

public class Test09 {
    
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
    
        Class C1 = Class.forName("TestReflected.student2");
        //通过反射获得注解
        //System.out.println(C1.getDeclaredAnnotations());
        Annotation[] annotations = C1.getAnnotations();
        for (Annotation annotation : annotations) {
    
            System.out.println(annotation);
        }

        //获得注解value的值
        TableYan tableYan = (TableYan)C1.getAnnotation(TableYan.class);
        String value = tableYan.value();
        System.out.println(value);


        //获得类指定的注解
        Field f = C1.getDeclaredField("name");
        FieldYan annotation = f.getAnnotation(FieldYan.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.length());
        System.out.println(annotation.type());

    }

}
@TableYan("db_Student")
class student2{
    
    @FieldYan(columnName = "db.id",type = "int",length = 10)
    private int id;
    @FieldYan(columnName = "db.age",type = "int",length = 10)
    private int age;
    @FieldYan(columnName = "db.name",type = "String",length = 10)
    private String name;

    public student2() {
    
    }

    public student2(int id, int age, String name) {
    
        this.id = id;
        this.age = age;
        this.name = name;
    }

    public int getId() {
    
        return id;
    }

    public void setId(int id) {
    
        this.id = id;
    }

    public int getAge() {
    
        return age;
    }

    public void setAge(int age) {
    
        this.age = age;
    }

    public String getName() {
    
        return name;
    }

    public void setName(String name) {
    
        this.name = name;
    }

    @Override
    public String toString() {
    
        return "student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface TableYan{
    
    String value();
}

//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface FieldYan{
    
    String columnName();
    String type();
    int length();
}